(1) Filed of the Invention
This invention relates to an apparatus and method of quickly changing reticles used to expose a layer of resist on a wafer without the need to remove the wafer from the wafer holder. More particularly the invention relates to a reticle holder, which can be positioned relative to a wafer, having a number of reticle subholders, which can be positioned relative to the reticle holder. Reticles are placed in said reticle subholders and can be individually selected.
(2) Description of the Related Art
With increasing chip sizes it becomes increasingly important to increase the field size and improve the image quality of the photolithographic images used in the manufacture of the chips.
U.S. Pat. No. 5,438,204 to von Bunau et al. describes a twin mask system and method for forming photolithographic patterns. Two or more copies of a mask pattern are used in forming the images. The two or more copies are axially displaced from one another along the radiation path and are in axial alignment with each other along the radiation path. This arrangement provides increased depth of focus.
U.S. Pat. No. 5,838,450 to McCoy et al. describes a mask alignment system and methods which achieves reticle to wafer referencing. The system and methods provide information for complete field-by-field alignment for both step-and-repeat and scanning exposure systems.
As the resolution of optical lithography imaging systems increases, the field size must also increase to achieve chips with higher circuit densities and more circuits per chip. This larger field size is achieved by a number of methods.
The field size of the imaging lens in a projection systems can be increased. However, it is difficult to simultaneously increase both the resolution power and the lax field size of modern high numerical aperture large-field-size imaging lenses.
The step-and-repeat imaging system can be replaced by a step-and-scan imaging system so that the field size can be increased in the scanning direction. However, in the step-and-scan system the field size becomes limited by the size of the reticles. It is difficult to change the size of reticles because this would require extensive changes in the entire mask making infrastructure. If larger reticles could be achieved increased problems due to defects, loss of critical dimension uniformity, and placement errors are likely to occur.
In light of these problems, it would be highly advantageous to be able to quickly change reticles during the exposure process. Quick changes of reticles would be an advantage in situations where multiple exposures are called for, when the wafer map consists of more than one reticle, to reduce the reticle changing time for increased wafer throughput, or the like.
It is a principle objective of this invention to provide an apparatus which permits quick changing of reticles used by an exposure system to expose a wafer without removing the wafer from the wafer holder.
It is another principle objective of this invention to provide a method for exposing a layer of resist on a wafer using an apparatus which permits quick changing of reticles used by an exposure system to expose a wafer without removing the wafer from the wafer holder.
These objectives are achieved by the use of a reticle holder having a number of reticle subholders. Each of the reticle subholders holds one reticle. The reticle subholders have manipulators on two or three sides so that each reticle can be positioned relative to the reticle holder. The reticle holder has two interferometer mirrors on each side so that the reticle holder can be positioned quickly with respect to a wafer using means such as a laser interferometer. Using the means to position the reticle holder and the manipulators on each of the reticle subholders new reticles can be quickly be positioned with respect to the wafer being exposed.